Method and apparatus for hot forming elongated metallic bars

ABSTRACT

A hot stretch wrap forming apparatus includes a die having a work surface formed of a thermally and/or electrically insulative material, and a set of spaced jaws for stretching and wrapping the metal form around the work surface. A preferred insulative material is a flexible blanket of woven ceramic fibers. A heat source is used to heat the metal form prior to stretching and wrapping. The metal form is preferably heated resistively to maintain a uniform temperature throughout the metal form. The die is typically formed primarily of metal and thus the insulative material thermally insulates the metal form from the metal of the die to prevent the formation of hot spots which would otherwise occur therebetween. The insulative material also electrically insulates the metal form from the metal of the die to prevent shunting therebetween.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to hot stretch-wrap forming of metal parts, most particularly elongated metal bars. More particularly, the invention relates to hot stretch-wrap forming of titanium, titanium alloys and similar metals which are typically difficult to stretch-wrap form. In particular, the invention relates to hot stretch-wrap forming of a metal form using a die having a thermally and electrically insulated work surface.

2. Background Information

The present invention relates to the hot stretch-wrap forming of elongated metallic parts which are formed at high temperatures, and in particular parts made of titanium alloys which are manufactured by extrusion, forging, rolling, machining or a combination of these processes. Titanium alloys have been widely used as aerospace materials due to their excellent mechanical and corrosion properties in combination with being comparatively light weight. However, it is well-known that titanium alloys are difficult to form in general and require heating to a substantial temperature in order to properly form such parts. Titanium alloys are highly desirable for use in contoured structural members of an aircraft, but the formation of such structural members has been very limited due to the lack of a suitable and economically feasible method of forming such contoured members. The demand for such parts has increased with the desire for lightweight and high strength structural components such as chords in advanced airplanes.

One process currently available for forming elongated titanium parts is known as “bump forming”. This process involves the heating of an elongated part in the furnace to a predetermined temperature at which time the part is removed from the furnace and placed on forming blocks of a forming press. The press applies a bending force which results in a localized deformation of the part. The temperature of the part quickly decreases during formation and the resistance to forming thus significantly increases. Thus, bump forming requires repeated heating cycles to complete the forming process, which is time consuming and costly. In addition, the bending moment that results from bump forming causes tensile stresses in the section of the part above the neutral axis and compressive stresses below the neutral axis which lead respectively to cracks and wrinkles in the part. The considerable stress gradient within the part makes it difficult to control the geometry of the formed part. In addition, the localized deformation caused by the complex stress state of the part promotes the development of significant residual stresses therein, requiring an offline stress relief treatment with an expensive fixture. Bump forming also suffers from the lack of a guiding tool to achieve the required contour without resorting to a trial and error method. It is also difficult to maintain the structural integrity of the cross section, for example, along angles between flanges and the like. Post hot sizing has been suggested to improve the dimensional integrity of the formed part. Finally, bump forming is not amenable to computer simulation.

While the general concept of hot stretch-wrap forming has been known for some time, known prior art methods are not suitable for economically forming parts made of titanium alloys or other materials which are difficult to form. U.S. Pat. No. 2,952,767 granted to Maloney discloses an apparatus for stretch-wrap forming an elongated bar which is heated by resistance heating and wrapped around a metallic die heated by conventional heating elements within the die assembly. A major problem with this configuration is the electrical shunting effect that occurs between the heated die and the metal part as they contact one another, which leads to local overheating and necking of the part.

U.S. Pat. No. 4,011,429 granted to Morris et al. noted the above shunting effect and sought to overcome this problem by heating both the die and the elongated metal part via resistance by electrically connecting the die and the metal part in parallel and heating them with the same voltage. Unfortunately, this configuration is not practical because the parallel heating of the die and part requires a complex and prohibitively expensive configuration of the die. In addition, this method requires preheating of the part to a temperature substantially below its forming temperature while the die is heated to the forming temperature so that only the contacting portion of the part is brought up to the forming temperature upon contact with the die, which results in a non-uniform yield strength between the contacting and non-contacting portion of the bar. Because the deformation process is not uniform, it is extremely difficult to maintain the structural integrity of the formed part and to minimize the development of residual stresses.

The present invention addresses these and other problems as will be evident from the subsequent description.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an apparatus comprising a die having a work surface which is formed of at least one of a thermally insulative material and an electrically insulative material; wherein the work surface is adapted for wrapping a metal form there around during hot stretch wrap forming of the metal form; a set of first and second spaced jaws adapted for gripping the metal form; wherein the jaws are movable away from one another whereby the jaws are adapted to apply a stretching force to the metal form; and wherein at least one of the set and the die is movable relative to the other of the set and die between a pre-wrapping configuration and a post-wrapping configuration whereby the relative movement between the set and die is adapted to wrap the metal form around the work surface of the die.

The present invention further provides a method comprising the steps of heating a metal form; applying a stretching force to the heated metal form; and wrapping the heated metal form around a working surface which is formed of at least one of a thermally insulative material and an electrically insulative material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagrammatic top plan view of the hot stretch-wrap forming apparatus of the present invention showing thermally and electrically insulative material within the die cavity and a metal bar prior to wrapping around the die.

FIG. 2 is a sectional view taken on line 2-2 of FIG. 1 showing two layers of insulative material within the die cavity.

FIG. 3 is a sectional view similar to FIG. 2 showing the metal bar inserted into the die cavity with the insulative material separating the metal die and metal bar.

FIG. 4 is similar to FIG. 1 and shows the jaws having moved from the starting position of FIG. 1 to the completed position of FIG. 4 to wrap the elongated bar around the work surface of the die.

FIG. 5 is a diagrammatic side elevational view of one of the jaws of the present invention.

FIG. 6 is similar to FIG. 3 and shows a single layer of insulation instead of a double layer of insulation.

FIG. 7 is similar to FIG. 6 and shows a metal form and die cavity having a different configuration.

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The hot stretch-wrap forming apparatus of the present invention is indicated generally at 10 in FIG. 1. Apparatus 10 includes a die 12 and a pair of spaced jaws 14 which are configured to clamp a metal form shown as an elongated metal bar 16 adjacent respective ends thereof in order to stretch metal bar 16 when heated and wrap it around die 12. Apparatus 10 is particularly useful for hot stretch-wrap forming of bars made of titanium alloys. Jaws 14 are attached to respective swing arms which are not shown but are well known in the art. Each of jaws 14 is in communication with an electrical power source 18 via conductors or wires 20 to form an electrical circuit for resistively heating metal bar 16. The exemplary embodiment of apparatus 10 shows die 12 in electrical communication with power source 18 via wires 22 to form an electrical circuit by which die 12 may be resistively heated if desired. In addition, optionally a plurality of heating elements 24 may be inserted into die 12 for the heating thereof. Die 12 includes a body having a die face or cavity-bounding surface 26 which defines a T-shaped die cavity 28 (FIG. 2). Surface 26 and cavity 28 have an arcuate configuration which extends from a first end 30 of die 12 to a second end 32 of die 12.

First or inner and second or outer layers 34 and 36 of insulation serve as a die face cover which is positionable between die face 26 and metal bar 16 during the hot stretch-wrap process. Layers 34 and 36 are shown disposed within cavity 28 with first layer 34 abutting surface 26 of die 12 in a substantially continuous manner from first end 30 to second end 32 of die 12. Second layer 36 likewise abuts first layer 34 in a substantially continuous manner from first end 30 to second end 32. Layers 34 and 36 conform to surface 26 and thus are of a generally T-shaped configuration. Second layer 36 defines a working surface 38 which abuts metal bar 16 during the wrapping process. As shown in FIG. 2, metal bar 16 has a T-shaped cross section which is of a mating configuration with the T-shaped cavity 28 and working surface 38. Work surface 38 defines a T-shaped working space 40 in which bar 16 is disposed during the stretch-wrap process.

More particularly, each of first and second layers 34 and 36 is most preferably formed of a thermally and electrically insulative material. Alternately, one of layers 34 and 36 may be formed of a thermally insulative material and the other may be formed of an electrically insulated material if desired. While it is preferred to provide thermal and electrical insulation between die 12 and bar 16, it is contemplated that only a layer of thermal insulation or only a layer of electrical insulation may be used depending on the circumstances.

Referring to FIG. 5, the configuration of each jaw 14 is described. Jaw 14 includes first and second spaced arms 42 and 44, first and second insulation members 46 and 48 respectively connected to arms 42 and 44 and disposed therebetween, and first and second gripping members 50 and 52 respectively connected to insulation members 46 and 48 and disposed therebetween. First insulation member 46 thus prevents electrical communication between first arm 42 and first gripping member 50 and second insulation member 48 likewise prevents electrical communication between second arm 44 and second gripping member 52. First gripping member 50 is in electrical communication with wire 20 and with metal bar 16 when clamping bar 16 so that first gripping member 50 is part of the electrical circuit previously discussed for providing the resistive heating of metal bar 16. The electrical scheme shown in FIG. 5 is for illustration and does not preclude connection of the wire 20 to the either the second gripping member 52 or to both gripping members 50 and 52.

In the exemplary embodiment, layers 34 and 36 are formed of a flexible refractory material. This allows layers 34 and 36 to easily conform to the shape of the die cavity. In addition, the use of such flexible layers allows for versatility in positioning the layers prior to the wrapping process. For example, priorto insertion of the metal bar into the die cavity, the layers may be disposed within the die cavity (as shown), wrapped around a portion or all of the metal bar, or simply suspended between the cavity and the metal bar so that insertion of the metal bar into the cavity presses the insulation material into the desired shape. Layers 34 and 36 are typically refractory ceramic blankets. One such suitable ceramic blanket is sold under the name Kaowool. Such ceramic blankets typically provide both the thermal and electrical insulative properties previously described and are formed of woven ceramic fabric or fibers. These flexible blankets are also easily removed from the die cavity or the metal bar when degraded to a degree such that they are no longer useful for the present purpose.

While such ceramic blankets are one form of a desirable insulative material, other suitable materials may be utilized which provide the thermal and/or electrical insulative properties needed for the present invention and which are capable of withstanding the heat and pressure utilized during the wrapping process.

The operation of apparatus 10 is described with reference to FIGS. 1-4. With reference to FIG. 1, power source 18 is operated to cause an electrical current to flow through metal bar 16 to resistively heat metal bar 16 to a desired predetermined temperature. Once this temperature is reached, jaws 14 apply an outward stretching force as indicated at Arrows A, that is, a longitudinal tensile force or strain. Meanwhile, die 12 may or may not be heated depending on the particular circumstances. If die 12 is to be heated, power source 18 may be operated to resistively heat die 12 via wires 22 and/or heating elements 24 may be heated to heat die 12. Whether or not die 12 is heated, jaws 14 are then moved toward die 12 to move bar 16 into working space 40 as indicated at Arrows B in FIGS. 3 and 4. Alternately or in combination, die 12 may be moved to facilitate the relative movement between die 12 and jaws 14. Jaws 14 are then moved as indicated at Arrows C in FIG. 4 to force bar 16 against work surface 38 of layer 36 to wrap bar 16 around die 12 to form the arcuate configuration of the formed part as shown in FIG. 4. The longitudinal stretching force continues to be applied to metal bar 16 during the wrapping of the bar around die 12. Thus, jaws 14 move from the pre-wrapping configuration of apparatus 10 shown in FIG. 1 to the post-wrapping configuration shown in FIG. 4.

The electrically insulative property of layer 34 and/or 36 prevents the electrical shunting between bar 16 and die 12 which was discussed in the Background section of the present invention. In addition, the thermal insulative property of layer 34 and/or 36 minimizes or eliminates the creation of hot spots in bar 16 which might otherwise be caused by die 12 when it is heated, and especially if not uniformly heated. The thermal insulative property also allows for the use of die 12 either without heating die 12 or heating die 12 at a substantially reduced level compared to known prior art configurations.

Thus, apparatus 10 provides a configuration for resistively heating metal bar 16 without creating shunting problems or hot spots during the wrapping operation. This is highly beneficial due to the fact that the resistive heating of metal bar 16 provides for uniform heating throughout the metal bar. Preferably, metal bar 16 is heated to a particular temperature range, is stretched at a relatively low strain rate and is held at the temperature range throughout the process and for a holding period after the wrapping process in order to allow formation of the metal bar in its final form substantially without springing back and substantially without undesirable tensile or compressive stresses within the formed part. Longitudinal stretching may continue during the holding period if desired. The uniform temperature of bar 16 is preferably maintained throughout the stretching and wrapping process and during any holding period, even if die 12 is not independently heated or is heated to a temperature substantially below the forming temperature of bar 16. Insulation layers 34 and/or 36 greatly facilitate the ability to maintain this uniform temperature due to the corresponding reduction in heat loss from bar 16 and prevention of the hot spots previously discussed. Apparatus and methods for several preferred aspects of the invention are described in greater detail in the copending patent application entitled Method And Apparatus For Creep Forming Of And Relieving Stress In An Elongated Metal Bar, which is filed concurrently herewith and incorporated by reference herein.

FIG. 6 shows an alternate embodiment in which only layer 34 of insulation is used, thus eliminating a layer 36. As previously discussed, layer 34 may have thermal and/or electrical insulation properties depending on the specific circumstances although typically both thermal and electrical insulative properties are desired.

FIG. 7 shows an alternate die 54 defining a U-shaped cavity for use with a generally U-shaped metal bar 56. First and second layers 58 and 60 are used in a manner analogous to layers 34 and 36 and conform to the U-shaped configuration of the cavity of die 54. FIG. 7 thus represents that the process can be used with metal bars of a wide variety of cross sectional shapes. Other shapes may have an outwardly-facing recess, such as a U-shaped configuration which is inverted relative to bar 56. These recesses may be filled with an elongated flexible structure to maintain the proper configuration during hot-stretch formation, for instance, to prevent the legs of the inverted U-shape bar from deforming toward one another.

In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed.

Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described. 

1. An apparatus comprising: a die having a work surface which is formed of at least one of a thermally insulative material and an electrically insulative material; wherein the work surface is adapted for wrapping a metal form there around during hot stretch wrap forming of the metal form; a set of first and second spaced jaws adapted for gripping the metal form; wherein the jaws are movable away from one another whereby the jaws are adapted to apply a stretching force to the metal form; and wherein at least one of the set and the die is movable relative to the other of the set and die between a pre-wrapping configuration and a post-wrapping configuration whereby the relative movement between the set and die is adapted to wrap the metal form around the work surface of the die.
 2. The apparatus of claim 1 wherein the work surface is formed of a thermally and electrically insulative material.
 3. The apparatus of claim 1 wherein the insulative material comprises a ceramic material.
 4. The apparatus of claim 1 wherein the insulative material comprises a flexible blanket of fibrous refractory material.
 5. The apparatus of claim 1 wherein the die includes a body having a die face and a cover which includes the work surface; and wherein the cover is positionable adjacent the die face whereby the cover is adapted to be positioned between the die face and the metal form.
 6. The apparatus of claim 1 wherein the die has a metal outer surface and a first layer of thermally insulative material positionable in abutment with or adjacent the metal outer surface whereby the first layer is adapted to reduce heat transfer between the metal form and the metal outer surface of the die during the hot stretch wrap forming of the metal form.
 7. The apparatus of claim 6 wherein the material forming the first layer is electrically insulative whereby the first layer is adapted to prevent electrical communication between the metal form and the metal outer surface of the die during the hot stretch wrap forming of the metal form.
 8. The apparatus of claim 6 wherein the die includes a second layer of electrically insulative material positionable at one of a first location between the metal outer surface and the first layer and a second location in which the first layer is disposed between the metal outer surface and the second layerwhereby the second layer is adapted to prevent electrical communication between the metal form and the metal outer surface of the die during the hot stretch wrap forming of the metal form.
 9. The apparatus of claim 1 further including a heat source adapted for heating the metal form to a forming temperature.
 10. The apparatus of claim 9 wherein the heat source includes an electric power source and a pair of electrical conductors in electrical communication with the power source; the electrical conductors adapted for electrical communication with the metal form for resistively heating the metal form.
 11. A method comprising the steps of: heating a metal form; applying a stretching force to the heated metal form; and wrapping the heated metal form around a working surface which is formed of at least one of a thermally insulative material and an electrically insulative material.
 12. The method of claim 11 wherein the step of wrapping includes the step of wrapping the heated metal form around a surface of a metal die with a layer separating the surface of the metal die from the heated metal form wherein the layer is formed of at least one of a thermally insulative material and an electrically insulative material.
 13. The method of claim 11 wherein the step of wrapping includes the step of wrapping the heated metal form around an outer surface of a metal die with a layer of electrically insulative material separating the outer surface of the metal die from the heated metal form to prevent electrical communication between the metal form and the metal die.
 14. The method of claim 11 wherein the step of wrapping includes the step of wrapping the heated metal form around a working surface which is formed of a material which is both thermally and electrically insulative.
 15. The method of claim 11 wherein the step of wrapping includes the step of wrapping the heated metal form around a working surface which is formed at least partially of a ceramic material.
 16. The method of claim 11 wherein the step of wrapping includes the step of wrapping the heated metal form around a working surface which is formed at least partially of a blanket of fibrous refractory material.
 17. The method of claim 11 wherein the step of heating includes the step of passing an electrical current through the metal form to heat the metal form resistively.
 18. The method of claim 11 further including the step of positioning a cover which includes the working surface between the metal form and a die face of a body of a die; and wherein the step of wrapping includes the step of wrapping the heated metal form around the cover and the die face.
 19. The method of claim 11 further including the step of inserting the metal form into a die cavity to press a flexible layer against a cavity-bounding surface which bounds the die cavity to conform the flexible layer to the cavity-bounding surface and to a mating outer surface of the metal form wherein the flexible layer is formed of at least one of a thermally insulative material and an electrically insulative material.
 20. The method of claim 11 wherein the step of heating includes the step of heating the metal form to a temperature which is substantially uniform throughout the metal form; and further including the step of maintaining the substantially uniform temperature throughout the step of wrapping. 